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European Journal of Heart Failure 2002 4(3):289-295; doi:10.1016/S1388-9842(01)00236-7
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© 2002 European Society of Cardiology

Assessing exercise capacity, quality of life and haemodynamics in heart failure: do the tests tell us the same thing?

Andrew R. Houghton*, Maxine Harrison, Alan J. Cowley and John R. Hampton

Department of Cardiovascular Medicine University Hospital, Queen's Medical Centre, Nottingham NG7 2UH, UK

* Corresponding author. Department of Cardiology, Glenfield Hospital, Leicester LE3 9QP, UK. Tel.: +44-116-287-1471; fax: +44-116-250-2405. E-mail address: arhoughton{at}btinternet.com


   Abstract
Top
 Abstract
1. Introduction
 2. Methods
 3. Results
 4. Discussion
 References
 
Background: The objective measurement of exercise tolerance is an important component of heart failure trials. The use of laboratory-based treadmill exercise testing has attracted criticism, however, as being unrepresentative of patients’ true capabilities.

Aim: To examine the relationships between tests of exercise capacity, quality of life and haemodynamics in patients with stable symptomatic heart failure.

Methods: Thirty-six patients with mild–moderate chronic heart failure were studied. Exercise capacity was assessed in the laboratory by maximal treadmill tests and self-paced corridor walk tests, and in the patients’ homes by hip-borne pedometers. Quality of life was assessed by a disease-specific questionnaire. Cardiac output and limb blood flow were measured by non-invasive techniques.

Results: Customary activity as assessed by pedometer scores correlated with quality of life questionnaire scores (rS=0.47, P=0.04), and both variables correlated with limb (calf) blood flow (pedometer scores: rS=0.39, P=0.03; quality of life scores: rS=0.50, P=0.04). The laboratory-based maximal treadmill test correlated with the self-paced corridor walk test, but neither of these tests correlated with pedometer scores, quality of life or haemodynamics.

Conclusions: Different methods of assessing exercise capacity do not appear to give comparable results and bear different relationships to haemodynamic variables and quality of life. Pedometer scores of customary activity may better reflect patients’ quality of life and appear to be more closely related to limb blood flow than the maximal treadmill exercise test or the corridor walk test. The sole use of laboratory-based exercise tests in therapeutic trials may give a misleading assessment of treatment efficacy in heart failure patients.

Key Words: Heart failure • Exercise capacity • Quality of life • Haemodynamics

Received June 7, 2001; Revised September 3, 2001; Accepted October 23, 2001


   1. Introduction
Top
 Abstract
 1. Introduction
2. Methods
 3. Results
 4. Discussion
 References
 
The objective measurement of exercise tolerance is a common and important component of treatment trials in heart failure and is frequently assessed by the laboratory-based maximal treadmill exercise test [1]. The use of maximal exercise tolerance as a study end point has attracted criticism, however, not least because it is a somewhat ‘unnatural’ form of exercise and may not measure the true capabilities of patients whose daily exercise patterns are unlikely to be reflected by the rigid requirements of the treadmill protocols [2,3].

Such limitations have led to interest in other, less artificial, methods of submaximal exercise assessment such as the 6-min walk test [4,5], the self-paced 100-m corridor walk test [6,7] and measurement of customary activity at home using pedometers [7–9]. Although submaximal exercise tests have intuitive appeal as being more representative of patients’ capabilities, it is not known how the results of these tests relate to maximal treadmill exercise time, quality of life or haemodynamic parameters in patients with heart failure. The aim of the present study is to investigate the relationships between these variables.


   2. Methods
Top
 Abstract
 1. Introduction
 2. Methods
3. Results
 4. Discussion
 References
 
2.1. Study population
The investigation conforms with the principles outlined in the Declaration of Helsinki. The local Ethics Committee had approved the studies and all patients gave their informed consent to participation. Thirty-six patients enrolled in clinical trials of heart failure medication were studied between 1994 and 1996. Although the patients were all participating in randomised treatment trials, all of the assessments were made during the run-in phases of these trials and the patients were therefore not receiving blinded trial medication at this time.

The patients had stable symptomatic heart failure and confirmed left ventricular systolic dysfunction on echocardiography (ejection fraction <40%, fractional shortening <25%, or visual assessment by an experienced echocardiographer). All the patients were taking a minimum dose of 40 mg frusemide (or equivalent) daily and had been on a stable dose of diuretic for at least four weeks. Exclusion criteria were haemodynamically significant obstructive valvular disease or recent myocardial infarction (within 12 weeks), stroke or symptomatic arrhythmia.

All 36 patients underwent assessment of haemodynamics (cardiac output and calf blood flow) and exercise capacity (corridor walk test and hip-borne pedometers). In addition, 20 patients underwent maximal treadmill tests and quality of life assessment by questionnaire. On assessment visits, patients attended a temperature-controlled laboratory (23–24 °C) in the morning following an overnight fast. Patients rested supine for at least 30 min before measurements were undertaken.

2.2. Haemodynamic measurements
Cardiac output was measured using the indirect Fick principle, monitoring respiratory gases with a mass spectrometer (Marquette, Jupiter, FL, USA) and using carbon dioxide as the indicator. The method correlates closely with cardiac output measurement by thermodilution [10]. Carbon dioxide production was calculated from minute ventilation and mixed expired carbon dioxide concentration, the partial pressure of carbon dioxide in pulmonary venous (systemic arterial) blood was derived from end tidal carbon dioxide concentration and the partial pressure in mixed venous blood was measured following a rebreathing manoeuvre. These three variables can then be used to solve the Fick equation to give a value for cardiac output in l/min, which was then corrected for body surface area to give the cardiac index. Skeletal muscle blood flow was measured in the left calf by venous occlusion plethysmography with mercury in silastic strain gauges, giving blood flow values in ml/100 ml per min [11]. With non-invasive haemodynamic measurements close attention must be paid to potential errors resulting from measurement variability. The non-invasive methods used for this study have all been well-validated and the coefficient of variation has previously been reported for patients studied in our laboratory as follows: cardiac output 7.8%, limb blood flow was 10.5% [12].

2.3. Corridor walk tests
After a rest period of at least 30 min, patients undertook a 100-m corridor walk test at self-selected normal walking pace. The time taken to complete the 100-m walk was recorded. This test has been used previously in studies of patients with heart failure [6,7,9].

2.4. Pedometers
Customary daily activity was assessed in the patients’ homes using a pair of hip-borne pedometers (Digiwalkers, Steiner, Brentford, UK) issued for a period of 2 weeks. The total count displayed by each pedometer is proportional to the number of movements of a spring-loaded pendulum displaced by vertical hip movements during walking, and thus is also proportional to the number of footsteps taken. The pedometers were calibrated before each use. These pedometers have previously been validated and used in studies of patients with heart failure [2,7–9].

2.5. Treadmill tests
For the 20 patients in whom treadmill tests were undertaken, a symptom-limited treadmill exercise test using a modified Bruce protocol was used. The slope of the treadmill was increased at 3-min intervals in 0°, 1.3°, 2.6°, 4.3°, 5.4° and 6.3° stages; the treadmill's speed was 2.7 km/h for the first four stages, 4.0 km/h for the fifth and 5.4 km/h for the sixth stage. Patients were only eligible for inclusion if their exercise tolerance was limited by dyspnoea or fatigue and if their treadmill exercise duration was consistent (defined as a variation of <30 s or 5% from the previous visit).

2.6. Quality of life
Quality of life was assessed by means of a self-administered, disease-specific questionnaire which has been developed and validated within our department and used in a number of studies in patients with heart failure [13–15].

2.7. Statistical analysis
Descriptive data are expressed as mean±standard error of the mean (S.E.M.). Non-parametric analytical methods were used throughout. The relationships between different variables were assessed using Spearman's rank correlation coefficient (rS). A P-value of <0.05 was regarded as statistically significant.


   3. Results
Top
 Abstract
 1. Introduction
 2. Methods
 3. Results
4. Discussion
 References
 
3.1. Patients
Table 1 shows the characteristics of the 36 patients studied. All of the patients studied had stable mild/moderate (New York Heart Association functional class II or III) heart failure. The mean (S.E.M.) daily dose of frusemide (or equivalent) was 85 (13) mg. Digoxin was being prescribed to 10 patients at the time of the study at a mean (S.E.M.) daily dose of 187.5 (21) µg. Not all the patients included in this study underwent treadmill tests or quality of life assessment; in addition, assessment of certain variables (e.g. calf blood flow) could not be undertaken in some of the patients for technical reasons. Thus not all the comparisons described below were undertaken in all 36 patients and so, where appropriate, the relevant n values are provided.


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  		Table 1 Baseline characteristics of study population

 
3.2. Relationships between different exercise tests
Fig. 1 shows a significant inverse correlation between total symptom-limited treadmill exercise time and the total time taken to walk 100 m along a corridor at a self-determined ‘normal’ pace (rS=–0.64, P=0.002, n=20). There was no correlation between treadmill exercise time and customary physical activity as assessed by pedometer score (rS=0.12, P=0.63, n=19), nor was there any correlation between pedometer score and the ‘normal’-pace corridor walk time (rS=–0.10, P=0.55, n=35).


Figure 1
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  		Fig. 1 Relation between ‘normal’-pace corridor walk time and treadmill exercise time.

 
3.3. Exercise capacity and quality of life
We found a significant positive correlation between total score on the quality of life questionnaire and pedometer score (Fig. 2; rS=0.47, P=0.04, n=19). In contrast, there was no correlation between the quality of life score and treadmill exercise time (rS=0.34, P=0.14, n=20), and the correlation between the quality of life score and ‘normal’-pace corridor walk time was just outside conventional levels of statistical significance (rS=–0.44, P=0.053, n=20).


Figure 2
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  		Fig. 2 Relation between pedometer score and quality of life score.

 
3.4. Exercise capacity, quality of life and haemodynamics
Fig. 3 shows a significant positive correlation between calf blood flow and pedometer score (rS=0.39, P=0.03, n=32), and Fig. 4 shows a correlation between calf blood flow and quality of life score (rS=0.50, P=0.04, n=17). There was no relationship between calf blood flow and treadmill exercise time (rS=–0.13, P=0.63, n=17) or ‘normal’-pace corridor walk time (rS=–0.12, P=0.51, n=33).


Figure 3
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  		Fig. 3 Relation between pedometer score and calf blood flow.

 


Figure 4
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  		Fig. 4 Relation between quality of life score and calf blood flow.

 
3.5. Cardiac index
Cardiac index showed no relationship with pedometer score (rS=0.22, P=0.20, n=35), treadmill exercise time (rS=0.14, P=0.57, n=20), ‘normal’-pace corridor walk time (rS=–0.17, P=0.31, n=36), or quality of life score (rS=–0.08, P=0.73, n=20).


   4. Discussion
Top
 Abstract
 1. Introduction
 2. Methods
 3. Results
 4. Discussion
References
 
The limitation of exercise capacity is a central component of the clinical syndrome of heart failure and so the assessment of exercise capacity remains an important part of its evaluation, particularly in the setting of therapeutic trials. Ideally, the assessment of exercise capacity should not only provide an objective measurement of functional status but also provide useful ‘surrogate’ information for haemodynamic changes and also information about prognosis.

Laboratory-based treadmill exercise tests have long been a cornerstone in the objective symptomatic assessment of heart failure patients participating in therapeutic trials. However, formal laboratory-based treadmill tests have their limitations. As they are a form of exercise that bears little relation to typical activity patterns, their usefulness as a true representation of patients’ abilities is questionable. Our finding that there was no correlation between patients’ treadmill exercise times and their pedometer scores, a measure of customary daily activity, supports this view. Moreover, in our study there was no relationship between maximal treadmill exercise capacity and quality of life scores, a finding that has also been reported by others [16]. Although many heart failure treatment trials have reported small increases in maximal exercise capacity, the value of this in the absence of simultaneous improvements in quality of life has been challenged [3].

Although treadmill exercise times did not correlate with pedometer scores, they did correlate with 100-m corridor walk test times. Corridor walk tests may be more representative of patients’ exercise capacity as they are self-paced, although they share the disadvantages of being laboratory-based and requiring patients to exercise in a manner that is somewhat artificial. Previous studies have examined the relationship between the corridor walk test and the maximal treadmill test. One study has shown no correlation between changes in corridor walk test times and changes in symptom-limited treadmill test times in response to treatment [6].

Our study revealed no relationship between treadmill times and cardiac index. Franciosa et al. also found no correlation between treadmill exercise duration and resting ejection fraction in a study of 21 patients with heart failure due to cardiomyopathy [17]. A similar lack of correlation between central haemodynamics and treadmill exercise capacity has been reported in other heart failure studies [18–20]. It has been suggested that blood flow to skeletal muscle is likely to be a more important determinant of exercise capacity than cardiac output, and previous work has shown no correlation between resting cardiac output and blood flow to skeletal muscle [21]. We were not surprised, therefore, to find that none of the exercise assessments correlated with cardiac index.

Pedometer assessments overcome many of the theoretical objections to laboratory-based exercise tests as they measure patients’ customary activity over a longer time period within their normal everyday environment. In addition to being a less artificial means of assessing exercise capacity, pedometer scores are also useful predictors of long-term survival in patients with chronic heart failure [7]. The weekly pedometer scores in almost all of our heart failure patients were below those seen in fit elderly controls, whose counts are 42 000–50 000 steps/week [22]. Unlike the laboratory-based exercise tests, the pedometer scores in our study correlated with quality of life questionnaire scores, suggesting that they provide a more reliable indicator of patients’ overall perception of their physical wellbeing. Our observation that patients’ pedometer scores bore no relation to their performance on laboratory-based exercise tests supports the view that these different exercise assessment modalities provide complementary information on different aspects of patients’ exercise capacity [2,9,15].

It was interesting to observe that both pedometer scores and quality of life questionnaire scores correlated with calf blood flow. As skeletal muscle blood flow is thought to be an important determinant of patients’ symptoms [21], this relationship is not entirely surprising. Those patients with greater skeletal muscle blood flow are likely to be capable of more daily activity and enjoy a better quality of life as a consequence. The failure of laboratory-based exercise tests to reflect this may limit their ability to give us a comprehensive picture of heart failure patients’ capabilities and their subsequent response to treatment.

4.1. Study limitations
A significant limitation of this study is its reliance upon multiple correlations. Where the interrelationships of several variables are examined in a study, there is always a risk that one or more ‘significant’ correlations are found purely by chance. One way to try to allow for this effect is to apply the Bonferroni method to the statistical analysis; in a study such as this which has examined 14 relationships between exercise, haemodynamic and quality of life variables, the Bonferroni method would suggest that correlations are only truly significant if a P-value of less than 0.05/14, or 0.0036, is attained. Applying the Bonferroni correction to our data indicates that the only relationship with a high likelihood of validity is the correlation between the treadmill exercise test and the normal-pace corridor walk test, which attained a P-value of 0.002. The Bonferroni method is a ‘conservative’ statistical method, but nonetheless the other correlations noted in this study should be considered with this limitation in mind.

4.2. Conclusions
In summary, our findings suggest that home-based customary activity as measured by pedometers correlates with quality of life questionnaire scores, and that both variables correlate with calf blood flow, in patients with symptomatic heart failure. The laboratory-based treadmill and ‘normal’-pace corridor walk tests correlate with each other but neither test appears to have any relationship with pedometer scores, haemodynamics or quality of life. In evaluating new pharmacological treatments for heart failure the sole use of laboratory-based exercise testing may lead to an incomplete assessment of their efficacy. The inclusion in such studies of pedometer and quality of life assessments should therefore be considered.


   References
Top
 Abstract
 1. Introduction
 2. Methods
 3. Results
 4. Discussion
 References
 

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